This application is related to Japanese Patent Application No. 2000-194325 filed on Jun. 28, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention:
The present invention relates to a heat-pump water heater that heats water using a heat pump cycle as a heating source. More particularly, the present invention relates to a heat-pump water heater which prevents a heat radiation from refrigerant to outside air in an air heat exchanger of a heat pump cycle.
2. Description of Related Art:
When freon (e.g., R22) is used as refrigerant in a heat-pump water heater, temperature of heated water is only increased to about 65xc2x0 C. In this case, because it is impossible to increase the water temperature to a high target supply temperature (e.g., 90xc2x0 C.) only using the heat-pump water heater, water heated by the heat-pump water heater is further heated using an electrical heater until hot water having the high-target supply temperature is obtained.
To overcome this problem, when carbon dioxide is used as refrigerant in a super-critical (trans-critical) heat-pump water heater, because a high-pressure side refrigerant pressure becomes higher as compared with a general heat-pump water heater using freon as refrigerant, the water temperature to be supplied can be increased to 90xc2x0 C. without using an electrical heater. However, in the super-critical heat-pump water heater, the high-pressure side refrigerant pressure of a heat pump cycle is controlled by an expansion valve so that a temperature difference between a temperature of refrigerant flowing out from a water heat exchanger and a temperature Tw of water flowing into the water heat exchanger becomes approximately constant (e.g., 10xc2x0 C.). Therefore, as shown by the p-h diagram in FIG. 7, when the temperature Tw of water flowing into the water heat exchanger is increased, the opening degree of the expansion valve is gradually increased for increasing the refrigerant temperature discharged from the water heat exchanger. When the opening degree of the expansion valve becomes larger, the high-pressure side refrigerant pressure Ph of the heat pump cycle is reduced, a water-heating capacity Q of the heat pump cycle is reduced, and it is difficult to increase the supply water temperature to the high-target supply temperature. Further, when the water temperature Tw flowing into the water heat exchanger becomes equal to or higher than 60xc2x0 C., the refrigerant temperature discharged from the water heat exchanger becomes higher, heat of refrigerant may be radiated to outside air in an air heat exchanger, and cycle efficiency of the heat pump cycle becomes lower.
In view of the foregoing problems, it is an object of the present invention to provide a heat-pump water heater for heating water using a heat pump cycle as a heating source, in which a heat radiation from refrigerant to outside air in an air heat exchanger is restricted so that a cycle efficiency can be improved.
According to the present invention, in a heat-pump water heater for heating water using a heat pump cycle as a heating source, a control unit controls operation of the heat pump cycle to set one of a general cycle operation where refrigerant flowing from a water heat exchanger is supplied to an air heat exchanger after being decompressed in an expansion valve, and a bypass operation where refrigerant from the water heat exchanger flows through the bypass passage to be sucked into a compressor while bypassing the expansion valve and the air heat exchanger. The control unit has a water temperature sensor for detecting a water temperature flowing into the water heat exchanger, and a heat-radiation determining member which determines whether or not a heat radiation from refrigerant to outside air will occur in the air heat exchanger based on the water temperature. In addition, the control unit performs the bypass operation when the heat radiation is determined by the heat-radiation determining member. Accordingly, in the air heat exchanger of the heat-pump water heater, heat radiation from refrigerant to outside air can be effectively restricted, and a cycle efficiency of the heat pump cycle can be improved.
Preferably, the heat-radiation determining member has first heat-radiation determining means for determining whether or not there is a possibility of the heat radiation from refrigerant to outside air in the air heat exchanger based on the water temperature, and second heat-radiation determining means for determining the heat radiation based on a comparison between a refrigerant temperature flowing into the air heat exchanger and a temperature of outside air flowing into the air heat exchanger when the first heat-radiation determining means determines the possibility of the heat radiation. Further, the control unit performs the bypass operation when the second heat-radiation determining means determines that the heat radiation will occur. Accordingly, the heat radiation can be accurately determined, and the heat radiation from refrigerant to air in the air heat exchanger can be further restricted.
Alternatively, the heat-radiation determining member has first heat-radiation determining means for determining whether or not there is a possibility of the heat radiation from refrigerant to outside air in the air heat exchanger based on the water temperature, and second heat-radiation determining means for determining the heat radiation based on a comparison between a refrigerant temperature flowing into the air heat exchanger and a refrigerant temperature flowing out from the air heat exchanger when the first heat-radiation determining means determines the possibility of the heat radiation. Further, the control unit performs the bypass operation when the second heat-radiation determining means determines that the heat radiation will occur. Accordingly, the heat radiation can be accurately determined, and the heat radiation from refrigerant to air in the air heat exchanger can be further restricted.